Thin polycrystalline film manganese doped zinc chalcogenide phosphors have been prepared by radio-frequency (rf) sputtering. In the conventional application of this technique, the phosphor is deposited upon a heated substrate in an rf electric field using either a powder or a solid hot-pressed powder target of the phosphor material in a low pressure inert atmosphere--usually of argon gas. Radio-frequency (rf) sputtering has considerable commercial attractions as a method for depositing thin films. However, it has been established that for the production of efficiently luminescent ZnS:Mn thin films rf sputtering is satisfactory only if followed by a high temperature annealing process. For example (see Cattell et al, Thin Solid Films 92 (1982) 211-217) it has recently been shown that, under cathodoluminescent excitation, the saturation brightness of conventionally prepared rf sputtered thin film phosphors on silicon substrates may be enhanced by a post-deposition anneal treatment. As there reported, a number of different phosphor samples were treated by raising the sample substrate temperature to one of several different peak temperatures 400.degree., 500.degree., 600.degree. and 700.degree. C. respectively and maintaining each sample at peak temperature for a prolonged period of time, usually 1/2 hour, before allowing each sample to cool naturally. This was done in a resistively heated tube furnace in a continuously flowing argon atmosphere. The reported results show that with this post-deposition anneal treatment, the saturation brightness is increased progressively with increased peak temperature attained, at least up to a temperature of 700.degree. C., appreciable increase in brightness being attained for temperatures in the range 600.degree.-700.degree. C.
Unfortunately, however, such post-deposition heat treatment is not readily applicable to electroluminescent panel manufacture. Such panels incorporate transparent electrode structures--eg electrodes of tin-oxide, indium tinoxide, or of cadmium stannate material. These electrode materials may become increasingly unstable when subjected to high treatment temperatures, ie, temperatures above 400.degree. C., for prolonged periods; and indeed with some substrates the glass softening temperature may be such as to limit heat treatment to 450.degree. C.
A solution to fabrication of a low cost high luminescent efficient ZnS:Mn film is not in itself sufficient for the fabrication of a successful low cost electroluminescent device. Such a device requires the non-destructive passage of high currents (.about./A/cm.sup.2, low duty cycle pulses for example) through the luminescent film and the background art consists of numerous partially successful schemes for providing this. In many, the solution has been to incorporate copper into the ZnS material but the inherent instability of Cu.sub.x S at temperatures above 60.degree. C. has led to undesirable long term degradation effects. In others, copper has been avoided by automatically limiting the destructiveness of high currents by the use of capacitative coupling wherein the active ZnS:Mn film is supplied with current through encasing insulator layers. These insulators pass only displacement currents and these die away before the breakdown of the ZnS film becomes destructive. This capacitative coupling technique (commonly referred to as `AC`) requires the use of an inconveniently high alternating drive voltage which leads to high cost.
A better solution is to use direct coupling and to combat the inherent tendency of the ZnS to break down destructively. Hanak (Japan J Appl Phys Suppl 2, Pt 1 (1974) 809-812) has shown that the use of a high resistance current limiting rf sputtered high resistance cermet film intermediate the phosphor film and the backing electrode enhances stability at the price of considerable I.sup.2 R losses in the limiting layer which leads again to examine drive voltage and loss of efficiency.